1. Field of the Invention
The present invention relates to a glow plug for assisting start-up of a diesel engine and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, a glow plug used for assisting start-up of a diesel engine has a metallic tubular metal shell, in which a rod-like heater is directly or indirectly held on a leading end side of its axial hole such that a leading end portion of the heater projects therefrom. In addition, a metallic rod-like center pole projects from a rear end side of the metal shell, and is held in the axial hole so as to be electrically insulated from the metal shell. A pair of electrodes for energizing the heater are electrically connected to the metal shell and the center pole, respectively.
The diesel engines in which glow plugs having the above-described structure are used are undergoing a shift toward direct injection type diesel engines in place of conventional auxiliary chamber type diesel engines. This is in the light of demands in recent years for features such as compact size, more efficient fuel consumption, and higher output. In addition, there are cases where the engine mounting structure is changing in conjunction with this trend, so that there is a need for compact-sized or elongated glow plugs. Further, there are frequent cases where ceramic heaters are used which provide high corrosion resistance.
As the overall length of the glow plug has increased, the natural frequency of the center pole has decreased. Consequently, instances in which the frequency of the vibrational load occurring upon operation of the diesel engine coincide with the natural frequency of the center pole have increased. Hence, a possibility has arisen that resonance can frequently occur which is undesirable. If resonance occurs, there is a possibility that a portion of the center pole corresponding to its vibration loop will come into contact with the inner peripheral surface of the metal shell, thus loosing the electrical insulating characteristic. In addition, the greater the amplitude the greater the deflection of the center pole, such that the center pole can possibly break. Furthermore, the ceramic heater may break due to internal stress transmitted from the center pole.
Accordingly, a glow plug has been proposed in which the lead members of the center pole are covered with an insulating coating to prevent short-circuiting between the center pole and the metal shell due to the resonance. Also, the outside diameter of a vibration preventing member is made close to the inside diameter of the metal shell so as to suppress the amplitude of the vibration loop, whereby the stress is alleviated to prevent breakage of the center pole (e.g., refer to JP-A-11-176563). If the stress occurring due to resonance of the center pole is reduced, it also becomes possible to prevent breakage of the ceramic heater.
3. Problems to be Solved by the Invention
However, compact metal shells are now used in conjunction with compact glow plugs, and the clearance between the inner peripheral surface of the axial hole and the outer peripheral surface of the center pole has become small. Consequently, in the process of manufacturing the glow plug it is difficult to insert an insulating coating as the vibration preventing member as proposed by JP-A-11-176563 into the axial hole of the metal shell with the center pole inserted therein. Accordingly, even if the center pole is covered in advance with the insulating coating, and the center pole is inserted into the axial hole, there is a possibility that the insulating coating whose thickness has been made thin to conform with the clearance becomes turned up and broken. Also, there is another problem in that even if the vibration preventing member is interposed in such a small clearance, since the thickness of the vibration preventing member itself becomes very thin, it is difficult to obtain a sufficient vibration preventing effect. Meanwhile, in the case where the outside diameter of the center pole itself is made small to secure the thickness of the vibration preventing member, its rigidity declines. As such, there is a possibility that when bending occurs due to stress caused by the resonance, the center pole becomes plastically deformed.
The present invention has been achieved to overcome the above-described problems, and an object thereof is to provide a glow plug which makes it possible to obtain a sufficient vibration preventing effect by interposing a vibration preventing member in the clearance between the center pole of a heater/center pole integrated member which is in a mechanically rigid state and the axial hole of the metal shell, as well as a method of manufacturing the same.